Transmission of data via optical fibers, serving as optical fiber waveguides, is widespread in many applications and industries, such as the telecommunications and computer industries. In many applications, electrical signals are converted into light pulses and propagated through optical fiber. The lights pulses leaving the optical fiber can then be converted back into electrical signals and processed.
Fiber optics has many advantages in many industries. For example, the use of optical fibers can result in significant improvements in bandwidth over more traditional means of communications. Another benefit is that optical fiber connections are far less vulnerable to electromagnetic disruptions and nuclear radiation, than other transmission mediums. In fact, fiber optics are now widely used in aerospace and shipboard applications for many of these reasons.
In many situations repair of optical fibers is required in the field. Such repair typically requires splicing fibers together using a splicer. Splicing operations in the field can be hampered by the environment and limited length of slack in the fiber that may be accessible for the technician to manipulate into a splicing device. Accordingly, there is a need for reliable portable fiber splicers that can be used in the filed, so are not overly large or cumbersome.
Optical fiber waveguides in common use share a number of structural features. The waveguide almost invariably comprises a thin, elongated fiber core responsible for conducting the light and at least one additional layer. Most often the fiber core is highly pure glass surrounded by a first and intimately bonded layer termed a cladding and an outer layer called a buffer. The cladding, usually also glass, has an index of refraction lower than that of the core to insure that light is constrained for transmission within the core by total internal reflection. Typically, the buffer is composed of plastic or polymer and serves to protect the inner layers mechanically and to prevent attack by moisture or other substances present in the fiber's environment. Commonly a plurality of individual fibers (in some cases as many as a thousand) are bundled together and enclosed in a protective jacket to form a cable.
Commonly used fibers may further be classified as multimode or single mode. Multimode fibers typically comprise cores having diameters of 50-62.5 μm but in some cases up to 100 μm. Single mode fibers generally have a much smaller core that may be 9 μm or less in diameter. The glass cladding diameter is most commonly 125 μm but sometimes is 140 μm (with a 100 μm core). The exterior diameter is largely a function of the buffer coating, with 250 μm most common, although some fiber coatings may be as much as 900 μm in diameter. Alignment of fibers is a crucial part of the preparation for any splicing operation, but is especially challenging for single mode fibers that have small core diameter. In order to produce a high quality, low-loss splice, the two opposing ends to be joined must be aligned laterally to within a small fraction of the core diameter. Of course, the smaller the fiber diameter, the smaller the allowed deviation from perfect abutting alignment that may be tolerated.
Most fiber optic data transmission systems transmit information using electromagnetic radiation in the infrared band, including wavelengths such as 850 nm for multimode fibers and 1310 and 1550 nm for single mode fibers. The nomenclature “light” is invariably employed for this radiation, even though the cited wavelengths fall outside the range visible to humans.
Mechanical and fusion splicing are the two typical approaches for splicing optical fibers. Mechanical splicing is accomplished by securing the ends of two fibers in intimate proximity with an aligning and holding structure. Often the fibers are inserted into the opposing ends of a precision ferrule, capillary tube, or comparable alignment structure. The fibers are then secured mechanically by crimping, clamping, or similar fastening. Mechanical splicing is conceptually simple, and minimal apparatus is required to effect splicing, but a mechanical splice tends to have relatively high and undesirable insertion loss, typically 0.20 dB. In addition, mechanical splices are generally weaker than the underlying fiber and are vulnerable to degradation of the optical quality of the splice over time, especially under adverse environmental conditions such as varying temperatures and high humidity. Mechanical splices are generally regarded as being temporary expedients at best and are not useful for high bandwidth systems or permanent joints.
Fusion splicing entails the welding of the two fibers, by softening and joining the ends of the fibers to each other. Heat is typically used to induce softening using a small electric arc struck between miniature pointed electrodes mounted in opposition and substantially perpendicular to the common axis of the fibers. Upon cooling, a strong, low-loss joint is formed. Fusion splices exhibit very low losses along with high stability and durability. A heat-shrinkable tube is typically applied over the completed joint for protection, which replaces a buffer coating removed prior to splicing.
For a low insertion loss splice the axes of the fibers must be collinear, within about 0.1 degree, and aligned laterally within a small fraction of the core diameter to achieve the desired loss of less than about 0.03 dB. This required precision of alignment presents a substantial technical challenge, especially with single-mode fibers having cores approximately 9 μm diameter. Three general approaches have been used. Mechanical fixturing can be used, such as the alignment ferrules and other forms of pre-aligned V-grooves and the like. These purely mechanical approaches do not reliably produce splices that maintain less than 0.10 dB loss.
More sophisticated approaches employ some form of optically assisted fiber positioning. For example, profile alignment system (PAS) is an approach where the splicing apparatus incorporates an optical system that acquires images of the two fibers taken in two lateral directions, allowing the fibers to be positioned in two directions orthogonal to the mutual fiber axes. PAS systems can incorporate either manual positioning or computerized image processing to optimize fiber alignment. However, the diffraction limit and the resolution of available electro-optic cameras restrict the precision achievable with PAS, even in systems based on visible light with wavelengths of about 400-700 nm. The effectiveness of PAS in aligning small diameter, e.g., single mode fibers, is limited.
Still more advanced positioning methods employ measurement of actual light transmission between the fibers being joined, where the positioning of the fibers is adaptively adjusted to maximize light transmission prior to the fusion operation. This approach can permit alignment better than that achievable with PAS systems, but requires carefully controlled laboratory conditions.
U.S. Pat. No. 7,077,579 to Bush, et al. described the use of piezoelectric bimorphs in fusion splicers as a means for aligning fibers to be spliced, as shown in FIG. 1 herein. Each side is coated with a single connected area, providing a continuous curve along the length of the substrate, as shown in FIG. 2. If one end is rigidly mounted, and voltages are applied to create a linear displacement of the distal end, a corresponding angular displacement I produced. If this angular displacement were conveyed to the optical fiber to be aligned, the angular displacement would produce unacceptable results. Therefore, prior art systems employ bimorph benders in pairs, connecting rigid blocks in a parallelogram arrangement as shown in FIG. 1. Blocks 8 and 9 are connected by a pair of piezo bimorphs. Flexible mounting arrangements (10a, 10b, 11a, 11b) are provided, to accommodate the continuously curved nature of the simple bimorphs. However, the need for connecting blocks, flexible mountings, and paired bimorphs adds to the cost, complexity, and difficulty of alignment using such prior art devices.
To date, the methods and apparatus for carrying out splicing aided either by the PAS, transmission-based alignment, and piezoelectric bimorphs techniques have not been well suited for use outside the laboratory or other similar workplaces. The required equipment lacks the flexibility, versatility, and ruggedness needed for field use. Moreover, present equipment is cumbersome and often not operable in the confined spaces frequently encountered during field service.